The subject matter herein relates generally to radio-frequency (RF) power generators and, more particularly, to RF power generators that utilize power electron vacuum tubes.
Radioisotopes (also called radionuclides) have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radioisotopes typically include a particle accelerator, such as a cyclotron, that accelerates a beam of charged particles (e.g., H− ions) and directs the beam into a target material to generate the isotopes. The cyclotron includes a particle source that provides the particles to a central region of an acceleration chamber. The cyclotron uses electrical and magnetic fields to accelerate and guide the particles along a predetermined orbit within the acceleration chamber. The magnetic fields are provided by electromagnets and a magnet yoke that surrounds the acceleration chamber. The electrical fields are generated by a pair of radio frequency (RF) electrodes (or dees) that are located within the acceleration chamber. The RF electrodes are electrically coupled to an RF power generator that may include, for example, oscillators, amplifiers, control circuitry, and power supplies. The RF power generator energizes the RF electrodes to provide the electrical field. The electrical and magnetic fields within the acceleration chamber cause the particles to take a spiral-like orbit that has an increasing radius. When the particles reach an outer portion of the orbit, the particles are directed toward the target material for radioisotope production. In addition to controlling the orbit of the particles, the RF electrodes may be used to pull the particles from a particle source in the acceleration chamber.
To operate the RF electrodes within the acceleration chamber, a considerable amount of electric power (e.g., 5 kW to 2 MW) is generated by the RF power generator and delivered to the RF electrodes. The power generator includes, among other things, a tube amplifier unit having a power electron vacuum tube (or power tube). The power tube is typically mounted to a grounding deck that separates a cathode cavity and an anode cavity. The power tube may be, for example, a power triode having a cathode, anode, and control grid. In a power triode, the cathode is heated by a filament that receives current from a power supply. The heated filament causes the cathode to emit electrons, which flow through the vacuum tube toward the anode. The control grid is positioned between the cathode and anode and may be used to control the flow of the electrons. Other power tubes utilized by power generators may include a tetrode and a pentode.
A socket is often used to mechanically and electrically couple the power tube to the grounding deck. The socket, however, can be expensive and can limit the options available for controlling the electrical performance of the RF amplification system. An alternative to the socket connection includes using flexible leads (also called flying leads) that are electrically coupled to the filament and are used to heat the filament during operation of the power tube. It can be a challenge, however, to connect the power tube to the grounding deck without a socket and be capable of operating within the very high frequency (VHF) band or higher while achieving a designated efficiency. For example, it is not recommended to operate power tubes with flexible leads at 100 MHz and above. Moreover, it may be desirable to monitor signals at the cathode resonator to control electrical performance. Socket-mounted power tubes, however, may require that a separate device be installed for monitoring the signals.